Problem: $ { {-5} \times \left[\begin{array}{r}{1} \\ {2} \\ {3}\end{array}\right] = {?} } $
Solution: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{r}{-5}\times{1} \\ {-5}\times{2} \\ {-5}\times{3}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{r}{-5} \\ {-10} \\ {-15}\end{array}\right]}$